This invention relates to a control valve, and in particular, to a high resolution control valve having a linear response.
U.S. Pat. No. 4,265,270 describes a control valve in which a rigid pivotally mounted arm is tilted by a stepping motor to open or close a gas valve. The mechanism for controlling the action of the arm is very complex and draws considerable power to operate. Because of the complexity of the operating mechanism, slight movement of the stepping motor produces relatively large movement of the valve seal. Accordingly, the resolution or control that can be exercised over the valve is relatively low.
There are some linear valves that have been devised which are solenoid actuated. Here again, power consumption is relatively high.
As will be described in greater detail below, the present control valve is ideally suited for use in a blood pressure monitoring instrument and, in particular, in association with an ambulatory blood pressure monitor system. In ambulatory units, which are typically worn by the patient over an extended period of time, the control valve must be light-weight, consume low power, yet have high resolution in order to closely control the positioning of the valve as the cuff is deflated so that accurate readings can be taken.
Solenoid actuated step valves are typically used in blood pressure monitoring devices due to their small size, lower power consumption and relatively low cost. These valves provide what is called a step deflation (see FIG. 7). In the simplest systems, a large step is required because of electrical, acoustic, and pneumatic noise due to the opening and closing of the solenoid valve. This noise interferes with the detection of blood pressure pulses or Korotkof sounds used to determine blood pressure. The larger the pressure drop per step, the less accurate the system. In a system such as this, inaccuracies can be overcome by using ECG gating. This, however, requires that electrodes be placed on a patient during monitoring so that heartbeats can be synchronized during valve operation so that the system noise due to valve operation does not interfere with the detection of blood pressure pulses or Korotkof sounds. This allows much smaller step sizes and, therefore, can be more accurate. This system, while enhancing accuracy, causes patient discomfort because of electrodes and wires, and increases the costs of systems and procedures.
Because of the difficulties using step deflation, an inherently more accurate method of determining blood pressure is to deflate a cuff in a linear fashion (see FIG. 7). This is the deflation method used when taking blood pressure manually, where the technician manually controls the deflation rate through a valve. The electronic components available to provide linear deflation in a portable blood pressure monitoring device are either too expensive, too large, and/or consume too much power during operation. Through the use of the valve of the present invention, linear deflation can be accurately controlled due to the high resolution of the valve.